A representative item or machine used in or by the construction industry is a backhoe. Backhoes normally consist of a boom that is pivoted on the frame of a tractor by a fluid ram or a hydraulic motor and an earth-working tool or bucket assembly that is pivoted on the free end of the boom by a second or another hydraulic motor. During transportation or storage, the boom and equipment attached thereto are positioned so as to occupy the least floor space. This makes movement on public roads and highways more convenient and safe since the center of gravity of the implement or backhoe is shifted closer to the wheels of the tractor. It also limits the backhoe tail swing which is of special concern when moving the backhoe over rough or uneven terrain.
A so-called "transport position" is generally disclosed by Long U.S. Pat. No. 3,376,984 which is assigned to the assignee of the present invention. In that patent, the boom is swung to a transport position that is generally vertical and slightly forward of the vertical axis extending through the boom and its support. That boom was held locked in the transport position by the boom fluid ram which had passed through its "over-center" position. While that arrangement is acceptable, there are times when because of age or excess wear, the hydraulic motor or fluid ram positioning the boom leaks to such an extent that the boom eventually slumps downwardly, particularly when the machine is moved over rough or uneven terrain. This is often referred to as "leak-down".
One method of positively interlocking the boom and its support without relying on fluid in the rams operating the boom is disclosed in Schumaker U.S. Pat. No. 3,811,582 (which is also assigned to the assignee of the present invention). While that method or scheme is satisfactory, it is particularly applicable to earth-working implements of a specific type (e.g., those with a fluid ram actuating a tool having a member pivoted intermediate its ends). It does not address ordinary hydraulically actuated pivoting arms and members such as a stabilizer assembly.
Thus, when it is necessary to move the machine over a relatively short distance or if the machine does not employ a positive mechanical interlock between the boom and the boom support, it would be desirable to have a hydraulic system which can make-up for whatever leakage is occurring at the hydraulic motor and thus prevent the boom or similar pivoted arm from slumping or slowly falling from an upright position.
This same problem is equally applicable to the hydraulic motors or actuators used to operate the stabilizer assemblies or outriggers. When the machine to which the stabilizers are attached is moved from one location to another, these stabilizer assemblies are normally raised or tucked inwardly so as to be clear of the ground. Some machines incorporate a mechanical lock to insure that the stabilizer assembly remains in its upright or raised position even if hydraulic fluid was lost from the hydraulic motor. It is particularly desirable to have something to solve this problem which is very low in cost and which incorporates equipment and systems already existing in the machine. Thus, if pre-existing equipment and systems can be made to serve a dual function, the utility of the overall system is enhanced at a relatively low cost.
The hydraulic system art is already aware of various arrangements for hydraulic pumps and valves for use in tractors or the like having hydraulic or power steering and hydraulic or power brakes and one or more valves for controlling the position of an implement such as a bucket. U.S. Pat. Nos. 4,010,610 and 4,005,636 by the present inventor describe two arrangements.
One device which has found particular usefulness in hydraulic systems used by backhoes, loaders and the like is a pressure compensating valve. These valves are often used so that a control valve or component in the system always has a constant pressure drop cross it to achieve smooth and precise hydraulic operation. The pressure compensating valve is hydraulically connected to a fixed displacement hydraulic pump. A biasing member together with a hydraulic line filled with fluid representative of the hydraulic load pressure is often used to establish a constant hydraulic pressure drop across the component or control valve supplying fluid to the load monitored by the pressure compensating valve.
Many hydraulic systems used in earth-working machines such as backhoes and the like incorporate a pressure compensating valve in conjunction with a power steering valve. The pressure compensating valve in these systems are often referred to as an "unloading valve." The pressure compensation valve smooths the operation and response of the power steering mechanism. Whenever the machine or tractor is turned, the pressure compensating valve senses and responds to the change in return hydraulic pressure due to the operation of the power steering valve. The result is that the power steering valve always has a constant pressure drop across it. Since the pressure drop across the power steering valve is constant, the rate at which the steering hydraulic motors move or travel is a function only of the rate at which the steering wheel itself is turned; therefore, the load on the system has no effect on the operation of the power steering mechanism (at least up to the limit of the pressure relief valve which may be employed with the system).
Since the power steering mechanism is so common to material handling machines, it would be desirable to use that system to the fullest and to maximize its utility, particularly with regard to the lead-down problem previously described. It would be especially desirable if the problem could be solved in such a manner that it could be applied to existing equipment without extensive modification or cost.